1. Field of the Invention
The present invention relates to an optical information recording-reproducing apparatus, which employs a solid immersion lens (hereafter referred to as an “SIL”) for recording information on or reproducing information from a recording medium. In particular, the present invention relates to servo-control of a gap or focus.
2. Description of the Related Art
To increase the recording density of an optical disk, generally, the light spot diameter on the recording face of the optical disk is made smaller, by shortening the wavelength of light used for recording and reproduction, and by increasing the numerical aperture (NA) of the objective lens. Conventionally, a so-called SIL is employed, and a front lens of an objective lens system is brought close to the recording face of a recording medium, to a distance of a fraction (e.g., one-half (½) or less) of the recording light wavelength, to increase the NA to be higher than one in the air.
A method for this is disclosed in detail, for example, in a document: Japan Journal Applied Physics, vol. 44, (2005) pages 3564-3567, “Near Field Recording on First-Surface Write-Once Media with a NA=1.9 Solid Immersion Lens”.
This method is also described in detail in a document: Optical Data Storage 2004, Proceedings of SPIE, vol. 5380 (2004), “Near Field Read-Out of a 50 GB First-Surface Disk with NA=1.9 and a Proposal for Cover-Layer Incident, Dual-Layer Near Field System”, and so forth.
The prior art techniques are described below with reference to FIGS. 11 to 14. First, the constitution of the optical pickup for the near-field recording in the first document above is described with reference to FIG. 11. A light beam having a wavelength of 405 nm emitted from semiconductor laser 1, a light source, is converted into a parallel beam by collimator lens 2, and is introduced to beam-shaping prism 3, to make the light quantity distribution isotropic.
The light beam is further allowed to pass through non-polarized light beam splitter (NBS) 4, polarized beam splitter (PBS) 7, and quarter-wave plate (QWP) 8, successively. The light beam, which is linearly polarized, is converted by the quarter-wave plate 8 to a circularly polarized light beam. A portion of the light beam reflected by non-polarized beam splitter (NBS) 4 is introduced to photodetector (LPC-PD) 6 for control of the emission power of semiconductor laser 1.
The light beam, having passed through quarter-wave plate 8, is introduced to expander lens system 9. Expander lens system 9 serves to correct spherical aberration caused in the objective lens or SIL, as mentioned later, and is constituted of two lenses: the distance between the two lenses is adjustable, to correct the spherical aberration. The light beam from expander lens system 9 is introduced to rear objective lens 10 of the objecting lens system.
The objective lens system is constituted of rear objective lens 10 and SIL (front objective lens) 11, and is mounted on a two-axis actuator (not shown in the drawing) to drive the two-lens system in the focusing and tracking directions. Hereafter, rear objective lens 10 is simply referred to as an objective lens, and the front objective lens is referred to as SIL 11.
FIG. 12 illustrates the convergence of the light beam by objective lens 10 and focusing to the bottom face of hemispherical lens, SIL 11. The light beam is introduced perpendicularly to the spherical face of SIL 11, and is focused on the bottom face through the same path as that in the absence of the hemispherical SIL. This achieves the effect of shortening the wavelength corresponding to the refractive index of SIL 11 and achieves an effect of a decrease of the light spot diameter.
Specifically, the light spot on the recording face of the optical disk 12 corresponds to N×NA, where N denotes the refractive index of the hemisphere lens and NA denotes the numerical aperture of objective lens 10. For example, a combination of objective lens 10 of NA=0.7 with hemispherical lens SIL of N=2 gives an effective NA (NAeff) of 1.4. The tolerable thickness error of SIL 11 is about 10 μm. Therefore, the device can be mass-produced readily.
The recording and reproduction with a light spot having a diameter corresponding to NAeff can be conducted by action of evanescent light from the bottom face of the SIL onto the recording face, only when the distance between the SIL bottom face and optical disk 12 is not longer than a fraction of the wavelength 405 nm of the light source, for example, in a short distance of 100 nm or less. To keep this distance, the gap is servo-controlled, as described later.
The optical system for the return path is described with reference again to FIG. 11. The light beam reflected by optical disk 12 becomes a reversed circularly polarized light beam, and is introduced to SIL 11 and objective lens 10, whereby the light is further converted to a parallel light beam. Then, the light beam is allowed to pass through expander lens system 9, and quarter-wave plate 8. Thereby, the light beam is converted to a linearly polarized light beam polarized in the direction perpendicular to that in the light beam projection path. This polarized light beam is reflected by PBS 7.
The light beam reflected by PBS 7 is allowed to pass through halfwave plate (HWP) 13 to turn the polarization plane by 45°. The S-polarized light component of the light beam is reflected by polarized light beam splitter 14 to pass lens 15, and is focused on photodetector-1 (PD-1) 16. From the optical signal detected by photodetector 1 (PD-1) 16, the information on optical disk 12 is reproduced as an RF output 17 by an information reproduction circuit (not shown in the drawing).
On the other hand, the P-polarized light component of the light beam, which has a polarization plane turned by 45° by halfwave plate (HWP) 13, is allowed to pass hrough polarized light beam splitter (PBS) 14, and is reflected by non-polarized light beam splitter 18. The reflected light beam is allowed to pass lens 19, and is focused on two-division photodetector-2 (PD2) 20. From the signal detected by the two-division photodetector-2 (PD2) 20, tracking error signal 21 is generated by a tracking error-detecting circuit (not shown in the drawing). According to tracking error signal 21, the tracking is servo-controlled by a tracking servo circuit (not shown in the drawing).
Of the light beam reflected by the bottom of SIL 11, the portion not totally reflected, corresponding to NAeff<1, is also reflected as circularly polarized light rotating in a direction reverse to the incident light beam, similar to the reflected light from optical disk 12. In the totally reflected light beam corresponding to NAeff≧1, a phase difference δ, shown by the equation below, is caused between the P-polarized light component and the S-polarized component, and the circularly polarized light beam is deformed to an elliptically polarized light beam.tan(δ/2)=cos θi×√{square root over ((N2×sin2θi−1)/(N×sin2 θi))}{square root over ((N2×sin2θi−1)/(N×sin2 θi))}  (1)
Therefore, the reflected light beam after passing through a quarter-wave plate contains a polarized light component of the same polarization direction as that of the incident light beam. This polarized light component is allowed to pass through PBS 7, reflected by NBS 4, allowed to pass through lens 26, and is focused on photodetector-3 (PD3) 27. The light quantity of this light beam decreases monotonically with the decrease of the distance between the SIL bottom face and the optical disk in the near-field region. Therefore, gap error signal 28 can be obtained from the signal detected by photodetector-3 (PD3) 27.
The distance between the SIL bottom face and the optical disk can be kept at an intended distance of not more than 100 nm by servo-control of the gap within a prescribed threshold. The servo-control of the gap is described in detail in the first document discussed above (that is, the “Near Field Recording on First-Surface Write-One Media with a NA=1.9 Solid Immersion Lens” article). Since this light beam is not modulated by the information recorded on the optical disk, the gap error signal can be obtained stably, regardless of the presence or absence of the recorded information.
In the servo-control of the gap, the overshoot should be less than the above-mentioned 100 nm. An overshoot of more than 100 nm will cause collision of the SIL against the optical disk. This will damage the SIL and the optical disk. To prevent the overshoot, as one method, the speed of approach of the SIL to the optical disk in the servo-control may be lowered. However, the lower approaching speed requires a longer time for the servo-control, and is not desirable, practically.
Japanese Patent Application Laid-Open No. 2005-209246, for example, discloses an apparatus to solve this problem. FIG. 13 illustrates the constitution of this apparatus. In FIG. 13, to start the servo-control of the gap, approach speed-generating circuit 108 outputs a drive signal to actuator driver circuit 106, to bring the object lens and SIL of optical pickup 102 close to optical disk 101. Optical pickup 102 in FIG. 13 may have a constitution similar to the optical system illustrated in FIGS. 11 and 12. Optical disk 101 in FIG. 13 corresponds to the optical disk 12 shown in FIG. 11.
During approach of SIL 11 to optical disk 101, a gap error signal generated by gap error-generating circuit 104 is input to comparator 107. Comparator 107 outputs a signal LOW to meet a gap error signal higher than a prescribed level Vth, and a signal HIGH to meet a gap error signal lower than the prescribed level Vth to switch 109.
When the SIL is detected to have approached close to the optical disk in a near field state, comparator 107 outputs a signal HIGH to turn on switch 109. Thereby, servo-control of the gap is started. In this process, the gap error signal is transmitted through phase compensation circuit 105, switch 109, and adder 130, to actuator diver circuit 106.
During the transition from the far field state to the near field state, approach speed-generating circuit 108 generates a signal in a waveform 142 shown in FIG. 14. The comparator outputs a signal in a waveform 141. Approach speed-generating circuit 108 is set, preliminarily, to produce output 142 to keep the voltage constant after the time t1 when the gap error signal comes equal to or lower than a prescribed level Vth. In such a manner, the actuator is set to generate the approach voltage at a constant level at the start of the servo-control of the gap. Thereby, the initial speed of the SIL at the start of the servo-control of the gap is controlled to be nearly zero, to servo-control the gap stably.
In the method of the aforementioned Japanese Patent Application Laid-Open No. 2005-209246, the approach voltage is set to control the speed of the SIL to be nearly zero at the start of the servo-control of the gap. However, in the servo-control with optical disk 101 rotated by spindle 103, it is difficult to keep the SIL speed to be nearly zero at the start of the servo-control, in consideration of swinging of the face of optical disk 101 and spindle 103.
With a disk face swinging, for example, the gap can come to a near field state before the time t1, or before the approach voltage comes to a prescribed constant voltage. In such a case, the non-constant output voltage of approach voltage-generating circuit 108 can start the servo-control of the gap in a stage of the approaching speed of not zero, or in a state of a high speed of the SIL relative to the optical disk face.
FIGS. 6A to 6D illustrate the servo-control of the gap started when the approach voltage is not constant in a ramp state and the gap error signal 62 comes to be lower than a prescribed level.
In the upper graph in FIG. 6A, the abscissa axis represents the time, and the ordinate axis represents the size of the gap. In the lower graph of FIG. 6A, the abscissa axis represents the time, and the ordinate axis represents the level of the actuator-driving signal 61 and the level of the gap error signal 62.
In the upper graph of FIG. 6A, the size of the gap is shown to vary in a sine curve shape before a start of the servo-control of the gap. This variation of the gap is caused by a swing of the optical disk, in spite of the movement of the SIL at a constant speed. Further, after the start of the servo-control of the gap, the actuator-driving signal is produced in a sine curve shape, owing to the gap servo-control following the face swing of the optical disk. Incidentally, numeral 63 in FIGS. 6A and 6C denotes the ON-OFF change of the loop; numeral 64 in FIG. 6B denotes the collision; numeral 65 in FIG. 6D denotes no collision; and numeral 66 in FIG. 6C denotes the change of speed.
In the servo-control method of Japanese Patent Application Laid-Open No. 2005-209246, the gradient of the ramped actuator-driving signal is constant before the gap error signal comes lower than a prescribed level, which is affected by the face swing. The SIL is allowed to approach the optical disk at a constant speed according to a ramp-shaped function, and the approach movement is stopped, and the servo-control of the gap is started at the time when the gap error signal comes to a prescribed level. According to this method, the gap size can become zero, or less, immediately after a start of the servo-control. This signifies collision in this method, the speed of the approach should be controlled to be sufficiently low. Further, the gap size for the prescribed level Vth of the gap error signal is as small as about 100 nm or less. Therefore, overshoot of 100 nm or more will cause collision of the SIL against the optical disk. This is disadvantageous.